The need for accurately measuring both the rate of material weight transported by a conveyor belt as well as the totalized weight that has been transported past the scale has long been recognized. Materials transported by conveyor belts and consequently needing to be measured range from huge flow rates encountered in the mining and aggregate production industries to comparatively miniscule flow rates in the sanitary foods and pharmaceuticals industries. Displayed flow rate is very useful and necessary to afford a plant operator the needed information in order to maximize his production rate by making appropriate changes as needed. Totalized weight is needed in order to provide data on plant yield and productivity over a period of time, such as one shift or one 24-hour day. Both of these data are generally available in currently available belt scales.
A related need for measuring flow rates of materials transported by conveyor belts is in a machine called a weigh belt feeder, where the purpose of this machine is to deliver a flow rate slaved to another requirement, such as feeding granulated coal to the steam boiler of an electric power plant. Here the demand for electric power dictates the coal flow rate. The belt scale plays the role of a flow rate measurement device so that the plant control system has the necessary information to change this flow rate as needed to meet the momentary and changing demand for electric power. Both the need for totalized weight and the need for flow rate require similar components for the belt scale--a mechanical structure equipped with one or more weight measuring devices that supports a short section of the loaded belt and produces a signal indicative of the magnitude of the load, a belt motion measuring device, and an electronic signal processor that combines the weight and motion signals and computes totalized weight, belt speed, and material weight flow rate.
All three sets of components contribute to the accuracy of the entire belt scale. However, it has generally been recognized that the greatest increase in belt scale accuracy requires design improvements of the mechanical structure with its weighing devices. At the same time, simplification of the mechanical structure has also received attention in the prior art as witnessed by U.S. Pat. Nos. 4,260,034, 5,111,896, and 5,296,654. These patents all show belt scales without a structural crossbar member that, if present, would span the conveyor from one side to the other. But simplification and reduction of the belt scale's mechanical structure is normally accompanied by increases in errors of the weighing performance. This mechanical structure and its attendant weighing device or devices will henceforth be called `weighframe` in this patent application. The weighing devices will be referred to as `load cells`. The assembly comprised of one horizontal roll (carry roll) and two inclined (troughed) rolls, and appropriate framework to connect all three rolls is variously called `idler` or `idler assembly` or `three-roll idler`. The particular idler supported by the weighframe is called the weigh idler.
At least two categories of belt scale accuracy are sought in the industrial application of belt scales: (a) highest accuracy for belt scales that are used to determine quantity of goods sold or exchanged; (b) lesser accuracy for belt scales that measure in-plant inventory and production rates. By far the largest number of belt scales are needed in category (b) where simplicity, general purpose usefulness, and low cost are the most important criteria. Accuracy most be preserved and improved if possible, but is not as demanding as in category (a). It is this later category that this invention addresses, both in terms of reducing mechanical structure as well as maintaining greatest possible accuracy while substantially achieving the goals of category (b). factors such as ease of installation, no interferences with existing cross-braces of the conveyor, ease of alignment, no moving parts in the weighing mechanism, long life, and general-purpose application while still maintaining a satisfactory level of accuracy are all important considerations to the user of a belt scale. The manufacturer of belt scales on the other hand has been traditionally concerned with ease of manufacturing, lowest production cost commensurate with intended accuracy, and ability to manufacture in advance of the specific order from a customer. Because conveyor belts are extant in many different widths, and a wide variety of full-scale material loading, a manufacturer has to inventory a very large number of weighframes if the design dictates that each belt width and each belt loading range requires a separate weighframe.
Eliminating the need for a crossbar would greatly enhance the ability to manufacture and stock this equipment in advance of orders for them because belt width would no longer be a defining specification. Furthermore, ability to configure the weight capacity of a belt scale after its manufacture and at the time an order is received is equally important in maintaining flexibility in manufacturing. This later criterion has often been achieved in the past at the time of installing the weighing device, called a load cell, with the appropriate capacity for a particular user's needs. But the former criterion--no crossbar--has been somewhat more elusive in the past. The reason for relatively few belt scale weighframes without crossbars is as stated earlier--accuracy is sacrificed when this particular simplification is adopted. Hence the need for a general purpose belt scale that does not require a crossbar and can be configured for different full-scale belt loading is very obvious, but accuracy must not be compromised if this type of belt scale weighframe is to be successful.
The weighframe cited in U.S. Pat. No. 4,260,034 and shown in FIG. 1 addresses the need for low-cost manufacturing by using a pair of weight sensing load cells as the primary and only connecting link between the idler roll that supports the belt and the conveyor structural members. Therefore, the load cells must sustain not only the vertical downward forces due to the weight of the material transported by the belt, but also the horizontal and lateral forces exerted by the roller axle bearing ends on the load cells. Additionally, the load cells also must sustain twisting forces (called torsion) created by the twisting motion of the longitudinal conveyor supports (called stringers). This twisting action is due to uneven thermal expansion of the stringers caused by the sun during the daytime hours. All these forces generally have non-negligible effects on the load cells, such as output signal changes that are not related to any change in the weight of the material carried by the conveyor belt. Thus errors arise in measuring the weight. An extreme effect of these outside forces and twisting motions is premature failure of the load cells.
The weighing structure of U.S. Pat. No. 5,296,654, shown in FIGS. 2 and 3, is another such attempt at providing a low-cost belt scale without a crossbar to the in-plant inventory market. Here the cantilevered beams labeled 44 to which the strain-gage load sensors are attached must sustain the entire array of forces and torsion loading applied by the idler roll assembly, Similar to the previously cited prior art, these disturbances arise from the horizontal travel of the conveyor belt and its material loading as well as thermal expansion effects. Therefore, the strain-gage load sensors are influenced by disturbance effects not related to or caused by the downward-acting weight of the material on top of the belt. Just as in the previous apparatus, the results are errors in the output signal and, in some instances, premature failure of the sensors themselves. claim 1 in the cited patent specifically refers to measuring only the stresses due to material weight on the belt and isolating the cantilevered beams from " . . . shifting, twisting, and movement of the belt conveying means stringers . . . ". But the structure proposed to solve these problems does not in fact isolate the cantilevered beams and therefore the sensing elements as well from these error-generating forces and motions. Also, the mounting elements required to support the cantilevered beams and to secure them to the conveyor stringers are not at all simple to manufacture because of their multiplicity of direction changes--laterally in from the stringers, then vertically rising, then longitudinally forward, then back down, then forward again to support the idler roll assembly.
Throughout all these direction changes, parallel surfaces between the stringer mounting location and the idler roll assembly support must be maintained at the risk of creating some unwanted torsional twisting of the cantilever beams to which the weight sensors are attached. Also, alignment of the neutral axis plane identified as 42 in FIG. 2 with the top of the carry roll is not correct. The correct alignment is the plane 120 passing slightly above the axle 116 of the carry roll 24 in FIG. 5. Also, there is no provision in this structure for adjusting the vertical height of the carry roll 24 to match the vertical heights of adjacent upstream and downstream carry rolls. This alignment is essential for accurate belt scale weighing. Because there is no such adjustment provision, the installer must place shims between the belt scale marked 38 in FIG. 3 and the conveyor stringers 30. This shimming procedure is tedious and time consuming--not recognized as a user-friendly design.
In summary, the prior art falls short of providing a no-crossbar conveyor belt weighframe that meets the combined user and manufacturer criteria of reliability, accuracy, and manufacturing simplicity.